Established river (fluvial) bedrock erosion models, including the saltation-abrasion model (SAM), consider only a single representative particle (grain) size. An underlying assumption in current mechanistic models of fluvial bedrock erosion is that the rate of erosion in a river channel is proportional to the energy delivered to the bed by moving particles. These models do not address the extent to which channel erosion depends upon the sizes of the grains that the river transports.

To investigate the importance of grain size on bedrock erosion rates in the field, Turowski et al. measured the energy delivered by particle impacts to an instrumented channel in the Erlenbach (Alder Creek) near Brunni, Switzerland. By combining methods for measuring impact energy with automatic sensing of grain size distributions, the researchers show that the largest grains are the most important contributors to bedrock erosion. The team found that although the largest grain size class, with a median grain diameter of 86 millimeters, contributes an average of 8.8% of the total bed load mass, it is responsible for 41.5% of the energy delivered to the bed, a disproportionate effect not captured by SAM.

The researchers suggest that one reason for this effect may be that SAM does not currently account for all the different ways in which the grains flow downriver, including rolling and sliding. In particular, the model does not account for larger grain sizes, which are less likely to be transported by short hops known as saltation.

The finding that grain size strongly controls energy delivery to the bed adds considerable complexity to modeling channel and landscape evolution. The team suggests that it may be possible to focus future modeling on rare extreme events, which govern long-term erosion because they alone transport the largest grains. In addition, because grain size distributions in mountain rivers are largely determined by hillslope processes, the authors conclude that there is a need to better understand their role, as well as the role of lithology, in determining grain size distributions. (Geophysical Research Letters, doi:10.1002/2015GL063159, 2015)

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